/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.flink.runtime.util;

import org.apache.flink.util.Preconditions;

import javax.annotation.Nonnull;

import java.io.Serializable;
import java.util.ConcurrentModificationException;
import java.util.Iterator;
import java.util.NoSuchElementException;

/**
 * This class implements a list (array based) that is physically bounded in maximum size, but can
 * virtually grow beyond the bounded size. When the list grows beyond the size bound, elements are
 * dropped from the head of the list (FIFO order). If dropped elements are accessed, a default
 * element is returned instead.
 *
 * <p>The list by itself is serializable, but a full list can only be serialized if the values are
 * also serializable.
 *
 * @param <T> type of the list elements
 */
public class EvictingBoundedList<T> implements Iterable<T>, Serializable {

    private static final long serialVersionUID = -1863961980953613146L;

    @SuppressWarnings("NonSerializableFieldInSerializableClass")
    /** the default element returned for positions that were evicted */
    private final T defaultElement;

    @SuppressWarnings("NonSerializableFieldInSerializableClass")
    /** the array (viewed as a circular buffer) that holds the latest (= non-evicted) elements */
    private final Object[] elements;

    /** The next index to put an element in the array */
    private int idx;

    /** The current number of (virtual) elements in the list */
    private int count;

    /** Modification count for fail-fast iterators */
    private long modCount;

    // ------------------------------------------------------------------------

    public EvictingBoundedList(int sizeLimit) {
        this(sizeLimit, null);
    }

    public EvictingBoundedList(EvictingBoundedList<T> other) {
        Preconditions.checkNotNull(other);
        this.defaultElement = other.defaultElement;
        this.elements = other.elements.clone();
        this.idx = other.idx;
        this.count = other.count;
        this.modCount = 0L;
    }

    public EvictingBoundedList(int sizeLimit, T defaultElement) {
        this.elements = new Object[sizeLimit];
        this.defaultElement = defaultElement;
        this.idx = 0;
        this.count = 0;
        this.modCount = 0L;
    }

    // ------------------------------------------------------------------------

    public int size() {
        return count;
    }

    public boolean isEmpty() {
        return 0 == count;
    }

    public boolean add(T t) {
        elements[idx] = t;
        idx = (idx + 1) % elements.length;
        ++count;
        ++modCount;
        return true;
    }

    public void clear() {
        if (!isEmpty()) {
            for (int i = 0; i < elements.length; ++i) {
                elements[i] = null;
            }
            count = 0;
            idx = 0;
            ++modCount;
        }
    }

    public T get(int index) {
        if (index >= 0 && index < count) {
            return isDroppedIndex(index)
                    ? getDefaultElement()
                    : accessInternal(index % elements.length);
        } else {
            throw new IndexOutOfBoundsException(String.valueOf(index));
        }
    }

    public int getSizeLimit() {
        return elements.length;
    }

    public T set(int index, T element) {
        Preconditions.checkArgument(index >= 0 && index < count);
        ++modCount;
        if (isDroppedIndex(index)) {
            return getDefaultElement();
        } else {
            int idx = index % elements.length;
            T old = accessInternal(idx);
            elements[idx] = element;
            return old;
        }
    }

    public T getDefaultElement() {
        return defaultElement;
    }

    private boolean isDroppedIndex(int idx) {
        return idx < count - elements.length;
    }

    @SuppressWarnings("unchecked")
    private T accessInternal(int arrayIndex) {
        return (T) elements[arrayIndex];
    }

    @Nonnull
    @Override
    public Iterator<T> iterator() {
        return new Iterator<T>() {

            int pos = 0;
            final long oldModCount = modCount;

            @Override
            public boolean hasNext() {
                return pos < count;
            }

            @Override
            public T next() {
                if (oldModCount != modCount) {
                    throw new ConcurrentModificationException();
                }
                if (pos < count) {
                    return get(pos++);
                } else {
                    throw new NoSuchElementException("Iterator exhausted.");
                }
            }

            @Override
            public void remove() {
                throw new UnsupportedOperationException("Read-only iterator");
            }
        };
    }
}
